Martensitic transformations general features

The input data for the model consist of the description of the lattice deformation and the choice of the slip system in the lattice-invariant shear. The model has successfully predicted the observed geometrical features of many martensitic transformations. The observed and calculated habit planes generally have high indices that result from the condition that they be macroscopically invariant. 

Martensitic transformations in alloys are essentially order-disorder displacive transitions that take place very rapidly, because atomic diffusion does not occur. The discussion of the formation of martensite in the Fe-C system, in Section 8.2.5, is an example. This transition is the transformation of a cubic phase containing excess carbon in interstitial sites into a tetragonal phase. As any one of three cubic axes can be elongated, three orientations of the martensite c axis can occur. This is a general feature of martensitic transformations and the different orientations that can arise are called variants or domains of the martensitic phase. These variants are simply twins (see Section 3.4.10). 

This is a general term which covers transformations which bear some similarity to twinning, but involve a change of crystal structure. The name comes from the well-known, important transformation which occurs when austenitic steel is rapidly cooled and forms the hard tetragonal metastable phase called martensite . These transformations can also occur on the deformation or cooling of many crystalline materials and all of these martensitic transformations share the similar feature that they take place with no long-range diffusion. 

This chapter begins with a general consideration of the crystallographic features of martensitic transformations. The principles are general, and thus detailed descriptions of the crystal struaures and substructures for individual alloy systems such as Ni-Al versus Cu-Sn are avoided. A brief survey of shape-memory phenomena within the framework of martensite crystallography is presented this subject and the various martensite crystal structures are presented in detail in Chapter 26 by Schetky in Volume 2. Martensitic transformations and shape-memory phenomena are common to many... 

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